Recently, non-volatile semiconductor memory devices capable of retaining data even when the power is turned off have been widely used. In a typical non-volatile flash memory device, a transistor which constitutes a memory cell has a floating gate or an insulating film which is called a charge storage layer. Data is stored by accumulating electric charges in the charge storage layer. The flash memory with the insulating film as the charge storage layer has a SONOS (Silicon Oxide Nitride Oxide Silicon) structure where the charge is accumulated in the charge storage layer inside the ONO (oxide film/nitride film/oxide film) film.
Further, a virtual ground flash memory device stores multiple bits of information in the ONO film of a single memory cell by switching a source region and a drain region formed in the semiconductor substrate. In the virtual ground flash memory, the effective channel length must be long enough to avoid a complementary bit disturb (CBD) in which multiple bits in the ONO film interfere. Consequently, miniaturization becomes difficult. A flash memory provided with a trench section in the semiconductor substrate, and an ONO film formed on both side faces of the trench section has been developed. Since a channel is formed along the trench section, CBD can be prevented and miniaturization realized.